1. Field of the Invention:
This invention relates to the processing of coal and similar materials and is particularly concerned with an improved process for the upgrading of heavy bottoms produced by the liquefaction of coal an related carbonaceous solids.
2. Description of the Prior Art:
Processes for the liquefaction of coal and similar carbonaceous solids normally involve contacting the feed material with a hydrocarbon solvent and molecular hydrogen at elevated temperature and pressure. This results in partial breakdown of the complex high molecular weight starting material into lower molecular weight hydrocarbon liquids and gases. These are recovered from the liquefaction effluent, leaving a heavy liquefaction bottoms product which normally boils in excess of about 1000.degree. F. and generally contains suspended solid residues. The liquefaction bottoms may constitute fifty percent or more by weight of the total liquefaction products.
A variety of different systems for upgrading liquefaction bottoms have been proposed in the past. Among the most attractive of these is pyrolysis of the bottoms to produce gases, additional hydrocarbon liquids, and coke, followed by steam gasification of the coke to form hydrogen and carbon monoxide for use as a fuel. Studies have shown that such an integrated process has many potential advantages over other processing systems, particularly if a catalyst is added to the coke to accelerate the gasification rate. The catalysts proposed for this purpose have normally been alkali metal compounds, particularly potassium carbonate. It has been suggested that alkaline earth metal compounds might be used to catalyze the gasification of carbonaceous solids but studies have repeatedly shown that such compounds are relatively ineffective. The impregnation of coke or char produced from liquefaction bottoms with calcium hydroxide or a similar alkaline earth metal compound normally produces only a small increase in the gasification rate compared to that obtained when potassium carbonate or a similar compound is used as a catalyst and hence use of the potassium compounds is normally more economical, despite their considerably higher cost.